Camera rotates on Z angle (rolls) during following spline
You've got at least two options to control the roll of your camera as it follows the path.


Any curve has not only a longitudinal direction, but also a twist, interpolated between its control points. You can see it  by looking at the changing plane of the arrows which constitute the curve's Normal display. In edit mode, you can modify the twist by selecting control points, and after hitting CtrlT, winding the mouse around the selection. The camera will follow the twist.
For better and perhaps more intuitive control of the roll of your camera between control points, you can create an empty, and assign a 'Locked Track' constraint to the camera, with Y as the direction, and the empty as the target.
The empty then becomes a handle for the up direction of the camera,(which now ignores the twist of the path). How you control the empty is up to you.You can animate it, leave it stationary a long way off, make it follow the path too, so it's always easy to reach... all sorts.